In this series, we will build a todo application.
To begin, we will go over a very basic way to build this application and revise as we gain more knowledge.
I suggest following along and if you get stuck, you can fork the code from the Code Sandbox
In the previous post, we created a very simple todo app that can add new todos. Lets now add a simple storage option for the todo list.
1. Adding the useEffect hook
We are going to use the useEffect hook to add to our application. This useEffect hook will be responsible to save new todos into localstorage.
see MDN docs on JSON.stringify
// useEffect to run once the component mounts useEffect(() => { // localstorage only support storing strings as keys and values // - therefore we cannot store arrays and objects without converting the object // into a string first. JSON.stringify will convert the object into a JSON string localStorage.setItem("todos", JSON.stringify(todos)); // add the todos as a dependancy because we want to update // localstorage anytime the todos state changes }, [todos]); Now this is what we should have:
// don't forget to import useEffect from react import { useState, useEffect } from "react"; import "./styles.css"; export default function App() { // need a state to keep track of todos const [todos, setTodos] = useState([]); // need state to keep track of the value in the input const [todo, setTodo] = useState(""); // useEffect to run once the component mounts useEffect(() => { // localstorage only support storing strings as keys and values // - therefore we cannot store arrays and objects without converting the object // into a string first. JSON.stringify will convert the object into a JSON string // reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify localStorage.setItem("todos", JSON.stringify(todos)); // add the todos as a dependancy because we want to update the // localstorage anytime the todos state changes }, [todos]); // function to get the value of the input and set the new state function handleInputChange(e) { // set the new state value to what's currently in the input box setTodo(e.target.value); } // function to create a new object on form submit function handleFormSubmit(e) { // prevent the browser default behavior or refreshing the page on submit e.preventDefault(); // don't submit if the input is an empty string if (todo !== "") { // set the new todos state (the array) setTodos([ // copy the current values in state ...todos, { // setting a basic id to identify the object id: todos.length + 1, // set a text property to the value of the todo state and // trim the whitespace from the input text: todo.trim() } ]); } // clear out the input box setTodo(""); } return ( <div className="App"> {/* create a form element and pass the handleFormSubmit function to the form using the onSubmit prop */} <form onSubmit={handleFormSubmit}> {/* create an input element - make sure to add the value prop with the state value passed in and the onChange prop to update the state every time something is typed in the input */} <input name="todo" type="text" placeholder="Create a new todo" value={todo} onChange={handleInputChange} /> </form> {/* create a ul to hold all of the list items */} <ul className="todo-list"> {/* map over the todos array which creates a new li element for every todo (make sure to add the "key" prop using the unique todo.id value to the li element) remember this is an array of objects - so we need to access the property "text" to get the value we want to display */} {todos.map((todo) => ( <li key={todo.id}>{todo.text}</li> ))} </ul> </div> ); 2. Restoring todos from localstorage (lazy initial state)
We are going to change the initial state to what's saved in localstorage.
see react docs on lazy initial state
see MDN docs on JSON.parse()
// because localstorage is synchronous - that could slow down the application // instead of using an just an empty array as the initial state - we can use a function in its place, // which will only be executed on the initial render const [todos, setTodos] = useState(() => { // get the todos from localstorage const savedTodos = localStorage.getItem("todos"); // if there are todos stored if (savedTodos) { // return the parsed JSON object back to a javascript object return JSON.parse(savedTodos); // otherwise } else { // return an empty array return []; } }); 3. Put it all together
Now we should have simple storage solution for our todos.
// don't forget to import useEffect from react import { useEffect, useState } from "react"; import "./styles.css"; export default function App() { // need state to keep track of todos // because localstorage is synchronous - that could slow down the application // instead of using an just an empty array as the initial state - we can use a function in its place, // which will only be executed on the initial render // reference: https://reactjs.org/docs/hooks-reference.html#lazy-initial-state const [todos, setTodos] = useState(() => { // get the todos from localstorage const savedTodos = localStorage.getItem("todos"); // if there are todos stored if (savedTodos) { // return the parsed the JSON object back to a javascript object return JSON.parse(savedTodos); // otherwise } else { // return an empty array return []; } }); // need state to keep track of the value in the input const [todo, setTodo] = useState(""); // useEffect to run once the component mounts useEffect(() => { // localstorage only support storing strings as keys and values // - therfore we cannot store arrays and objects without converting the object // into a string first. JSON.stringify will convert the object into a JSON string // reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify localStorage.setItem("todos", JSON.stringify(todos)); // add the todos as a dependancy because we want to update the // localstorage anytime the todos state changes }, [todos]); // function to get the value of the input and set the new state function handleIInputChange(e) { // set the new state value to what's currently in the input box setTodo(e.target.value); } // function to create a new object on form submit function handleFormSubmit(e) { // prevent the browser default behavior or refreshing the page on submit e.preventDefault(); // don't submit if the input is an empty string if (todo !== "") { // set the new todos state (the array) setTodos([ // copy the current values in state ...todos, { // setting a basic id to identify the object id: todos.length + 1, // set a text property to the value of the todo state and // trim the whitespace from the input text: todo.trim() } ]); } // clear out the input box setTodo(""); } return ( <div className="App"> {/* create a form element and pass the handleFormSubmit function to the form using the onSubmit prop */} <form onSubmit={handleFormSubmit}> {/* create an input element - make sure to add the value prop with the state value passed in and the onChange prop to update the state every time something is typed in the input */} <input name="todo" type="text" placeholder="Create a new todo" value={todo} onChange={handleInputChange} /> </form> {/* create a ul to hold all of the list items */} <ul className="todo-list"> {/* map over the todos array which creates a new li element for every todo (make sure to add the "key" prop using the unique todo.id value to the li element) remember this is an array of objects - so we need to access the property "text" to get the value we want to display */} {todos.map((todo) => ( <li key={todo.id}>{todo.text}</li> ))} </ul> </div> ); } This is the second post in this series. Keep in mind that in this post, we added slightly more functionality to the app. We will continue to add more functionality in the coming posts.
Thanks for reading!
Top comments (7)
How to check if data is exist in localstorage before save the data?
@fbryo we do that when we set the state and use a function instead of an initial state value. Hope that helps!
But I still save the data, the purpose is I want to check if data is exist before save the data
Here's my modified code:
Looks like you are checking if the data exists here, keep in mind that this is getting an item from localstorage. We are saying, if there is data in local storage then alert "data exists":
If you want to check if data exists first, before saving to localsotrage - there are a few ways you can do it. Check in the
useEffecthook or in your example, possibly:Hope that helps!
Sorry, my bad. I revised my problem. The actual problem is, if selected cat/index equals to existed data, shows an alert. And else, it can be saved
hi man, I just want to say thanks. it works!
and easy to understand by beginner like me. thank again!